This invention relates broadly to parboiled rice products and to methods for producing parboiled rice. More particularly, the present invention relates to a parboiled rice product exhibiting greatly improved steamtable quality, characterized by an enhanced resistance to breakdown. Methods of the invention relate to rice parboiling processes sequentially involving soaking, steaming, drying and tempering stages.
As will be appreciated by those familiar with rice processing methods, the broad concept of parboiling rice to preserve its nutritional integrity and resistance to insect invasion during long storage periods has been known and applied for hundreds of years. The main advantage of the parboiling process is the resulting retention of nutrients and minerals in the starchy center of the rice. Over the years, it has been demonstrated that parboiling also results in a grain which is more easily dehulled, is more resistant to breakage during milling, has a higher percentage of superior quality bran oil, and withstands longer cooking times without becoming undesirably sticky.
Broadly, parboiling comprises the steps of soaking the harvested grains in water to increase the moisture content, steaming the moistened grains under pressure, and subsequently drying the rice prior to storage and milling. The soaking and steaming steps result in swelling and restructuring of the starch granules or starch-lipid complex in the rice grain. Research indicates that the overall minimum moisture content achieved during soaking and steaming is roughly 30% dry solids basis (d.s.b.) Drying typically comprises a plurality of drying phases during which the rice temperature and moisture content are gradually reduced so that heat stresses on the rice are minimized. During drying, the rice endosperm hardens, making the grain more resistant to breakage during milling. Milling removes the outer hull and bran layers from the starchy center of the rice grain.
Prior parboiling practices can be divided into three broad categories: the "conventional" method, the dry-heat method, and the pressure-steaming method. The conventional method comprises soaking, draining, steaming at atmospheric pressure, followed by drying and milling stages. The dry-heat method replaces the steaming step of the conventional method with a heating stage, in which the rice is cooked in dry, hot air or sand prior to drying. Finally, the pressure-steaming method comprises a low moisture initial soak (roughly 25-67% d.s.b. moisture content) followed by pressurized steaming prior to drying and milling. Various combinations of these individual steps have been applied in the art.
Parboiled rice finds a ready market in both large food service organizations, and in the home. Simple parboiled rice usually requires relatively long cooking times, which is a disadvantage for home use where "quick cooking rice" is favored. To produce quick cooking rice, an "instantization" process is required after parboiling, raising the cost of the product. Larger institutions, however, do not require quick cooking rice, since they have more than adequate time to slow-cook traditionally parboiled rice.
As will be appreciated by those skilled in the food service arts, in larger food service organizations such as institutions, cafeterias and the like, foods such as rice are commonly pre-cooked in volume and then transferred to steamtable pans. Typically, the pans are held in hot steam carrier cabinets or "carters" at temperatures of between 70-105 degrees Celsius prior to serving. When serving commences, the heated pan will be transferred to a steamtable and maintained at serving temperatures typically between 50-70 degrees Celsius.
Many relatively large scale food service operations have found it convenient and profitable to reduce labor-intensive individual service by abandoning a-la-carte offerings in favor of self-serve "buffets" or cafeterias. In such establishments a variety of conventional steamtables are quite commonly used for temporarily storing food, while maintaining the desired temperature. Even in "full service" restaurants or diners steamtables are often employed adjacent a salad bar, and their use provides customers with a convenient "self service" option. While such arrangements no doubt offer many advantages, product losses as a result of food deterioration over time can often result.
A clear advantage of steamtables is that while the food remains hot for serving, the food tends not to drY out in the heat. But conventionally processed parboiled rice may degrade measurably in response to steamtable overexposure. As will be appreciated, however, steam introduces water to the food product, and foods continue to slowly cook during steamtable storage. Since hot foods served in a typical luncheon buffet may remain over burners or in steamtable trays for a full two to three hour serving period, steamtable foods eventually tend to become soft and mushy, overcooked, and tasteless. Conventional rice processed by typical prior art parboiling methods is particularly vulnerable to steamtable degradation.
The focus of many prior art rice processing developments discussed below has been to produce a quick-cooking rice product for home consumption. Processes for "instantizing" rice require that the rice be essentially fully cooked after milling and redried (whether made from raw white or parboiled rice), resulting in substantial grain breakage and increased energy costs. As a result instantized rice is more expensive than mere parboiled rice. "Instant" or "quick-cooking" rice products are capable of rapid rehydration, so that cooking times are meaningfully shortened. While not a major consideration for large food service operations, a short cooking time is extremely important, for example, to one preparing a typical family meal for a small or limited number of diners. Food products which are capable of "instant" cooking are in great favor for home-prepared family meals. The shorter the cooking time of the various menu items, the easier it can be for the head of the household to coordinate the meal and serve all items concurrently. Rice provides a tasty and convenient side dish and thus quick-cooking rice products are in great demand.
But instantized rice which may be ideal for home preparation is not optimum for larger food services employing steam tables. Such rice is more expensive than simple parboiled rice, and its primary consumer advantage, that of "quick cooking," is not a benefit to food service establishments which typically have several hours to prepare their food. Further, we believe that many instantized rice products are at least somewhat susceptible to steam table degradation. Moreover, the known prior art has not attempted to customize a parboiled rice product which is immunized from the above discussed phenomena of steam table degradation. After relatively short periods of steamtable exposure, even quick-cooking rice may partially degenerate, as multiple kernels fall apart and clump together in an unappealing, soft, "pasty" mass. Over time the steamtable storage of rice typically results in the loss of consumer-appealing color, texture, and flavor.
As a result of steamtable degradation, rice is wasted. For large volume cafeterias, for example, cumulative food service losses engendered by wasted, steamtable-degraded products can be prohibitive. Hence, it is desired to produce a rice product which maintains an appealing light color and a chewy, non-sticky texture after extended periods of steamtable storage.
Recent research efforts to improve parboiled rice products have been directed at the modification of the rice starch structure. Modifying the grain starch in various ways may result in desirable product qualities, such as quick-cooking, freeze-thaw stability, and resistance to breakdown or "mushiness" during cooking. Native starch exists in two related forms in the rice granule, as amylose, or the "straight-chain" form and as amylopectin, or the molecular "branch" form. The greater percent of native rice starch is amylopectin. In the past, chemical treatments have been used to modify the native starch structure to achieve different qualities.
It has also been found advantageous to force gelatinization of rice starch granules. For purposes of this discussion, "gelatinization" refers to the disruption of the crystalline structure of the rice starch, usually as a result of the addition of water by soaking and steaming. In effect, gelatinized starch granules are "melted" together into an amorphous mass. Ungelatinized, crystalline starch is generally white in color. Most prior art parboiling methods attempt to avoid any significant gelatinization in the soaking phase and to vary steaming time, pressure, and temperature in order to thereafter effectuate the desired degree of gelatinization. As demonstrated in the specific examples discussed hereinafter, different processing methods achieve varying degrees of gelatinization of the native rice starch. As revealed in the prior art discussed hereinafter, at least some degree of gelatinization of the rice starch is desired in order to provide rice with improved kernel integrity when cooked.
Modified or gelatinized starch may also "reassociate" or recrystallize in different forms. In essence, the starch is crystallized from its gelatinized state to a strongly angular starch crystal structure. The extent of starch reassociation depends on both the moisture content and the temperature of the stored rice. As will be demonstrated in the specific examples hereinafter, reassociation forms new starch linkages which substantially affect product stability.
In this context, the term "stability" refers generally to the resistance of rice to the release or "leachout" of free starch during and after cooking. "Kernel integrity" broadly refers to maintenance of firm and compact individual grains throughout cooking. Typically, kernels of rice cooked or steamed for long periods of time will split open and fall apart. As the starch is freed or released from the kernel, the rice becomes sticky or pasty and may mass into unmanageable clumps.
Rice stability and kernel integrity can be measured through the use of various subjective standardized taste and visual tests after cooking. "Cooked quality" of the rice is measured immediately after cooking, and includes analysis of the rice texture, color, amount of free starch or stickiness of the cooked rice, and kernel integrity. The scores are commonly averaged together to obtain an overall average cooked quality score. Similar standard tests are conducted to determine "steamtable quality". Steamtable quality as used herein is measured after the rice has been exposed to water and heat on the steamtable for a predefined period. The scores for texture, color, free starch, kernel integrity, and overall steamtable quality can be readily compared to cooked quality scores. Various qualities of a rice product can also be predicted prior to cooking through the use of other laboratory procedures.
The degree of starch gelatinization and the amount of starch reassociation in a rice granule may be revealed with the use of a differential scanning calorimeter (DSC). DSC scans are used widely in the plastics industry to measure endothermic and exothermic characteristics of materials at given temperatures. Amylopectin starch is a naturally occurring high polymer, and thus naturally lends itself to DSC testing. A rice sample subjected to a DSC scan displays a heat flow curve, which can be compared to standard curves for interpretation of the internal structure and characteristics of the rice starch.
All of the above standard tests commonly applied in the food industry facilitate comparison of rice samples produced under different processes. The food service industry, however, is now most interested in consistent performance of the rice product on the steamtable. Additionally, the food canning and freezing industries may benefit.
A number of prior art methods are directed to the production of "quick cooking" or rehydratable rice which overcome problems resulting from undesirably long cooking times, particularly associated with "brown" rice products. One such "instantizing" method is disclosed by McCabe in U.S. Pat. 3,959,515, issued May 25, 1976 and comprises alternating soaking and baking stages. Soaking the rice results in starch gelatinization and increased grain size. Baking at temperatures of approximately 300 degrees F. dries the grains for storage and provides desirable color quality.
Other methods for producing "precooked" or quick-cooking rice products are described by Lou in U.S. Pat. No. 4,521,436, issued June 4, 1985; Barry, U.S. Pat. No. 4,333,960, issued June 8, 1982; and Kohlwey, U.S. Pat. No. 4,649,055, issued Mar. 10, 1987. The last two references disclose an additional puffing stage in which the rice is expanded under high heat.
The parboiling process set forth in U.S. Pat. No. 4,810,511 issued to Velupillai on Mar. 7, 1989 is directed mainly to reduction of energy expenditures and resultant product costs. The initial soaking stage results in a 26-32 percent (wet basis) water content. Thereafter the rice slurry is exposed to microwave energy (heated) for partial gelatinization to roughly forty percent water content. The rice is then drained and microwaved a second time to a water content of roughly fourteen percent The process results in a rice product which is substantially completely gelatinized, and purportedly resulted in higher than average milling yields.
Taniguchi, in U.S. Pat. No. 4,794,012 issued Dec. 27, 1988, proposes a method for producing a pregelatinized rice which can be stored for long periods and quickly cooked, for example, in a microwave oven. The method comprises successive soaking steps during which temperatures are gradually raised to prevent undesired putrefaction and to increase uniform moisture distribution throughout the rice grain. The rice is then steamed and boiled, resulting in gelatinization, with a moisture content of 45-75 percent w.b. Finally, the grains are dried and puffed to reduce the moisture content to roughly eight percent.
The aforementioned prior art references are generally directed to producing quick-cooking or instantized rice products. Increased starch gelatinization is achieved during initial steaming stages. Based on our experimentation, none of the above-referenced methods is capable of economically producing a rice product which is suitably resistant to steamtable breakdown. Moreover, none of the above-addressed prior art suggests effective means for both increasing rice kernel integrity and controlling energy expenditure and resultant production costs.
One reference of relevance to the present process and product is U.S. Pat. No. 4,361,593, issued Nov. 30, 1982 to Brooks. It is noted that the process defined therein is directed to the production of a parboiled rice suitable for subsequent instantization. Briefly, the '593 process comprises an initial soaking step, i which the moisture content is raised to 30-45 percent w.b.; a high-pressure steaming step, in which the degree of gelatinization is closely restricted; and an extended, controlled "tempering" step, which purportedly reduces the amount of free starch in the rehydrated rice. In this context, "tempering" generally refers to a "resting" period in which starch molecules are permitted to associate into water-resistant bonds at lower temperatures, averaging roughly 100 degrees Celsius. The tempering step purportedly hardens the starch in the rice and assures a non--sticky, quick--cooking rice product. After the tempering step, the rice is slowly dried to a moisture content of under fifteen percent w.b.
In U.S. Pat. No. 4,361,593 drying does not precede tempering. In column 5, lines 56-60, the latter reference states that "...Where the moisture content is reduced too rapidly to too great an extent...the tempering will not proceed to the proper degree and the product will tend to be relatively more starchy." The prior art thus teaches away from our concept, since we have found that a predrying step prior to tempering can be critical to the production of a parboiled rice product of enhanced steamtable quality.
Based on experimentation, however, it has been demonstrated that thus restricting gelatinization substantially reduces kernel integrity. In addition, tempering moist rice at high temperatures actually retards starch reassociation. While the process purportedly favors the production of a quick-cooking rice, it is demonstrated incapable of producing a high-stability, steamtable rice product, such as is currently in commercial demand.
Importantly, it is our experience that tempering the rice directly after steaming is commercially impractical. For example, the rice of the '593 method is immediately tempered after steaming. However, when very moist rice is stored for tempering as taught, the rice tends to release starch and "ball up" into sticky masses which cannot be conveyed or handled conveniently as individual or separate grains. Additionally, while such a high moisture tempering process may be easily carried out in the research laboratory, its translation to the production environment is highly impractical. Special equipment and processing required to accomplish such tempering would greatly raise production costs and render the rice end product too expensive for commercial purposes. In addition, the '593 process tends to reduce milling yields, which in turn further increases production costs.
Thus it is desired to provide a rice parboiling process which provides substantially complete gelatinization and favors starch reassociation whereby to yield a high-integrity kernel. No teaching is found in the prior art known to us for an economically feasible process for simultaneously achieving substantially complete gelatinization and controlled tempering for attaining high starch reassociation. None of the known prior art discloses adequate means for efficiently producing a rice product which retains kernel integrity as well as desirable color, texture, and taste qualities during extended periods of steamtable storage.